NFCRC Tutorial: Molten Carbonate Fuel Cell (MCFC)

The electrolyte typically
consists of a combination of alkali (Na and K) carbonates retained
in a ceramic matrix of LiAlO2. The cell operates at temperature
of 1100 to 1300 deg F or 600 to 700 deg C in order to keep the
alkali carbonates in a highly conductive molten salt form, the
carbonate ions providing ionic conduction. The anode is made from
Ni while the cathode is made from nickel oxide.

Three corporations actively
pursuing the commercialization of MCFCs in the U. S. are Energy
Research Corporation, International Fuel Cells Corporation, and
MC Power Corporation, in Europe are Brandstofel Nederland (BCN),
Deutsche Aerospace AG, Ansaldo (Italy), and in Japan are Hitachi,
Ishikawajima Harima Heavy Industries, and Mitsubishi Electric Corporation.

Note that CO is not
directly used by the electrochemical oxidation, but produces additional
H2 by the water gas shift reaction:
CO + H2O = H2 + CO2.

A fuel such as natural gas is either reformed
externally or within the cell in the presence of a suitable catalyst
to form H2 and CO by the reaction: CH4 + H2O = 3H2 + CO.
Any sulfur compounds present in the fuel have to be removed prior to
use in the cell (upstream of the reformer) to a concentration of <0.1
ppmV. The fuel cell itself, however, can tolerate a maximum of 0.5
ppmV of sulfur compounds.

Typically the CO2 generated at the anode is recycled
to the cathode where it is consumed. This requires additional equipment
to either transfer the CO2 from the anode exit gas to the cathode
inlet gas or produce CO2 by combustion of the anode exhaust gas and
mixed with the cathode inlet gas.

One of the advantages
of the high operating temperature of the MCFC is that the overall
thermal efficiencies is high, with a potential of 50 to 60% conversion
of the fuel (natural gas) LHV to electricity without recovery and
conversion of the exhaust heat. Also, the exhaust heat from the
MCFC is at relatively high temperatures (1200 deg F or 650 deg
C) and may be recovered for the generation of steam which further
increases the efficiency. Efficiencies >60% may potentially
be achieved with the incorporation of a bottoming cycle. On the
other hand, the higher operating temperature places severe demands
on the corrosion stability and life of cell components.

The method for electrolyte
management in an MCFC in order to establish a stable electrolyte/gas
interface in the porous electrodes depends on a balance in capillary
pressures to establish the electrolyte interfacial boundaries allowing
the electrolyte matrix to remain completely filled with the molten
carbonate, while the porous electrodes are partially filled, depending
on their pore size distributions.